As an important part of a wireless communications system, an antenna is a system component for radiating and receiving electromagnetic waves. Performance of the antenna decides performance of a mobile communications system. A high-performance antenna meets a requirement of a broadband system and improves performance of the entire system. A core problem of design of a modern antenna is to enable the antenna to meet more rigorous technical requirements in a new system, and surpass an original antenna form to meet new system requirements. With a rapid growth in a quantity of mobile users, the communications system is continuously updated and expanded. To reduce interference between antennas and to lower costs, the antenna is required to work within a broadband range, and meet requirements of communication between multiple systems at the same time, thereby achieving sharing of one antenna in multiple systems and sharing of one antenna in receiving and sending. A research in a base station antenna shared by multiple systems can reduce a quantity of antennas so as to reduce interference between the antennas and lower costs, and an original base station can be shared. Therefore, the research in a multi-band base station antenna unit is of great significance.
A base station antenna mostly uses a linear polarization manner. A monopole antenna mostly uses vertical linear polarization. A dual-polarized antenna generally includes two manners: vertical and horizontal polarization and +/−45-degree polarization. Generally, the latter has better performance than the former. Therefore, the manner of +/−45-degree polarization is used in most cases currently. Because one dual-polarized antenna consists of two mutually orthogonal polarized antennas packed in a same radome, use of the dual-polarized antenna can dramatically reduce a quantity of antennas, simplify antenna engineering and installation, lower costs, and reduce space occupied by an antenna, and is a mainstream of current antenna deployment in urban areas. The dual-polarized antenna combines two antennas whose polarization directions: a +45-degree direction and a −45-degree direction are mutually orthogonal, and the two antennas simultaneously work in receiving and sending duplex mode. In addition, because polarization is performed in the +45-degree direction and the −45-degree direction that are orthogonal, it can be ensured that a degree of isolation between the +45-degree antenna and the −45-degree antenna meets a requirement of intermodulation on a degree of isolation between antennas (>30 dB), so that spacing between dual-polarized antennas needs to be only 20 to 30 cm, and a good effect of diversity reception can be effectively ensured.
For conventional+/−45-degree polarized antennas, no relationship exists between radiation arms that correspond to a +45-degree polarization and a −45-degree polarization. When a radiation arm that corresponds to one polarization works, a radiation arm that corresponds to the other polarization does not work. When the conventional+/−45-degree polarized antennas are used to construct a plane array, a location and a feeding manner of a low-frequency unit cause significant impact on an adjacent high-frequency unit.